The invention relates generally to pressure sensors and more particularly to pressure responsive variable parallel plate capacitive transducers. Such transducers are shown and described, for example, in U.S. Pat. No. 4,716,492, assigned to the assignee of the present invention. A capacitive pressure transducer is shown having a thin ceramic diaphragm mounted in closely spaced, sealed, overlying relation to a ceramic base, with metal coatings deposited on respective opposing surfaces of the diaphragm and base to serve as capacitor plates arranged in predetermined closely spaced relation to each other to form a capacitor. Transducer terminals connected to the capacitor plates are arranged at an opposite surface of the transducer base and a signal conditioning electrical circuit connected to the transducer terminals is mounted on the transducer. A cup-shaped connector body of electrical insulating material is fitted over the electrical circuit and is secured to the transducer by a housing sleeve which has a port for exposing the transducer diaphragm to an applied pressure. The diaphragm is movable in response to variations in pressure applied to the diaphragm to vary the capacitance of the capacitor in accordance with the changes in applied pressure and the electrical circuit provides an electrical output signal corresponding to the applied pressure.
In order to maximize the economies of mass production to lower the transducer cost and thereby make such transducers economically feasible for a wide number of applications including many previously served by low cost mechanical transducers, a standard size package is selected small enough to be received in a large number of applications yet large enough to provide a reliable signal. The size of the package determines the maximum size of the capacitor plates which, along with the gap between the plates determines the capacitance signal. This results in limiting the size of the capacitor plates to a smaller size than would be ideal for many applications and relying on the electrical circuit to properly condition the signal. The circuit on the other hand requires a minimum level of capacitance for it to be able to effectively condition the output signal and this in turn affects the distance or gap required between the capacitor plates to produce the minimum capacitance level. In transducers of the type disclosed in U.S. Pat. No. 4,716,492 distances between the plates are in the order of 10-17 microns.
One approach described in the above patent to provide this selected gap employs a cup-shaped member having a relatively rigid rim secured to a base substrate disposing a bottom of the cup in selected, spaced overlying relation to a capacitor plate on the base substrate. An electrically conductive layer is disposed on the inner surface of the cup bottom to provide the second capacitor plate with the bottom being resiliently flexible to serve as a diaphragm to move the second plate toward and away from the first capacitor plate in response to variation in fluid pressure applied to the outer surface of the cup bottom. The configuration of the cup-shaped member, however, is not conducive to low cost mass manufacturing techniques. Due, in part, to the small sizes involved it is very difficult to obtain consistent flat surfaces on the cup bottoms which are parallel to the substrate surface. Slight variations from device to device cause changes in the capacitance signals produced by the transducers which frequently fall outside the window of values acceptable by the signal conditioning circuitry.
Another approach described in the above patent employs a flat diaphragm element secured to the base substrate in selectively spaced relation thereto by disposing a spacing and securing medium such as a mixture of glass frit including a plurality of balls of glass of selected diameter between the flat diaphragm and the substrate at the periphery of the diaphragm. The glass frit is selected to be fusible at a first temperature at which the balls remain unfused and the mixture is then heated to the fusing temperature of the frit to secure the diaphragm to the substrate at a spacing from the substrate determined by the diameter of the balls. The provision of flat surfaces which extend over the entire diaphragm as well as the base substrate is very conducive to consistent, reproducible results from device to device, however, the use of the glass material to both space and secure the diaphragm to the base substrate results in problems relating to the effectiveness of the seal between the diaphragm and the base substrate. Although the reason is not completely understood some devices are made which have imperfect seals. During manufacture the diaphragms are biased against the glass by means of a clip to provide a compressive force to cause the diaphragm to move toward the base as the glass melts when the temperature is raised. It may be that the glass balls prevent the surfaces of the diaphragm and the base substrate from being sufficiently close to one another to be wet by the glass at areas of imperfection in their surfaces. That is, although the surfaces are ground uneveness sometimes occurs due to imperfections in the grinding process. In any event it is found that the effectiveness of the seal lacks desired consistency.